WL#5295: Plugin API Documentation

This task serves as a placemarker related to server worklogs that have to do with
the plugin API, created so that it can be referred to in documentation planning.
Some of the relevant server WLs:
WL#3771: Pluggable Audit Interface
WL#4903: Plug-in Service API: part 1
WL#2940: plugin service: error reporting
WL#3961: Plugin Service: mysys examples
Some WLs deal with writing plugins that access the server. Others deal with
services provided by the server that can be used by plugins.